This invention relates to an image forming apparatus and, more particularly, to an image forming apparatus capable of setting a desired copy ratio.
Recently, various electronic copiers capable of copying in reduced or enlarged sizes have been placed on the market.
The operational principles of the electronic copier will now be described with reference to FIG. 1. An original A set on an original table 10 is scanned for exposure by a scanning unit consisting of an exposure lamp 12 and a mirror 14. Light reflected from the original A is directed by mirrors 16 and 18, a lens 20 and a mirror 22 to a uniformly precharged rotating photosensitive drum 24. An electrostatic latent image is thus formed on the photosensitive drum 24. It is then developed, and the visible image (i.e., developer image) thus formed is transferred onto a copying sheet P to complete one copying cycle.
As for the copy size, the dimension of the copy in the scanning direction of the original, is determined by the ratio between the scanning speed of the scanning unit and the rotational speed of the photosensitive drum 24 (hereinafter referred to as speed ratio). The dimension of the copy in the direction perpendicular to the scanning direction is determined by the ratio between the distance a of the optical path from the original A to the lens 20 and the distance b of the optical path from the lens 20 to the photosensitive drum 24 (hereinafter referred to as optical path distance ratio). Of course the relation 1/a+1/b=1/f must be satisfied, where f is the focal distance of the lens 20.
The original A can be copied on a desired scale, enlarged or contracted, by varying the speed ratio and optical path distance ratio.
However, the mechanism for continuously varying the speed ratio and optical path distance ratio is complicated, and in the currently available copier the copy size can be enlarged or contracted only in a particular ratio. In other words, it sometimes happens that the copy size cannot be enlarged or contracted to the size of a given copying sheet P. If the size of the original is too small, it can be copied only on a portion of the copying sheet even on the limit enlarged scale. On the other hand, if the size of the original is too large, it cannot be fully copied on the copying sheet even on the limit contracted scale.